The phosphate mining basin of Gafsa is part of the south-western dry area of Tunisia. It covers 3250 km(2), representing 42% of the entire governorate of Gafisa area and 38% of its total population. The area is characterised by water resources scarcity as well as high variability of rainfall. The water demand is in continual increase due to many factors such as the industrial, agricultural and socio-economic development. The confrontation between available resources and water demand for all sectors in the phosphate basin shows that the current global balance is in surplus. Taking into account the fact that all the sectors have neither the same requirement of water quality nor the same added value of the pumped water, it seems then necessary to review the current resources allowance model in order to establish rational exploitation plans so as to face the water demand increase. In the local context of the Gafsa phosphate mining basin, the treated waste water seems to constitute potential additional water resource that should be considered to reduce the groundwater resources overexploitation.